I own both and have used both and tested both on a 5d2 extensively. Price aside
Samyang:
smaller and lighter
Much less flare prone, much,
Much less CA (hardly any) though this is mostly correctible.
ACR profile available to autocorrect distortion
Sharper in the corner wide open, until about f5.6.
dont have to fuss with an adaptor, with all the potential issues (decentering) that it can produce.
Nikon
Zoom, useful
Slightly better microcontrast, if its not ruined by flare.
slightly shaper corner to corner at f8.
Far less distortion, though less easy to correct.
If you are looking primarily at 14mm, its a no brainer. Samyang is the better choice. Flare issues with Nikon are pretty bad, and the the sun does not have to be even in the field. samyang has exceptional sharpness by any standard. Nikon is legendary, b/c it matches the sharpness of best primes in its range, and its a zoom.
Edit
If you are using it on a7r, not sure if acr profile is available, but at least with samyang you wont have to worry about an adapter. with nikon you do
However, again, nikon is a zoom, and there is great utility to that. Fred of Fredmiranda fame is using the a7r extensively with his adapted canon TS lenses and seems to be very happy. I think its an excellent choice as a camera if autofocus is not an issue to you, and you want to continue to use your canon lenses.